The sense of touch has traditionally been regarded as one of the five classical senses, but in reality it is highly complex, transducing a number of different sensations. These sensations are detected in the periphery by a variety of specialised nerve endings and associated structures. Some of these are specific for mechanical stimuli of various sorts such as touch, pressure, vibration, and the deformation of hairs or whiskers. Another class of nerves is able to detect temperatures, with different fibres being activated by heat and cold. A further population of nerve endings is not normally excited by mild stimuli, but by strong stimuli only. Sensory nerves of this category often respond to more than one stimulus, and are known as high-threshold polymodal fibres. They may be used to sense potentially damaging situations or objects. The polymodal fibres also transduce chemical signals such as the “burning” sensation evoked by acid. Thus, the sense of touch can transmit a very detailed description of objects and serve to both inform and warn of events.
The transduction of sensory signals from the periphery to sensation itself is achieved by a multi-neuronal pathway and the information processing centres of the brain. The first nerve cells of the pathway involved in the transmission of sensory stimuli are called primary sensory afferents. The cell bodies for the primary sensory afferents from the head and some of the internal organs reside in various of the ganglia associated with the cranial nerves, particularly the trigeminal nuclei and the nucleus of the solitary tract. The cell bodies for the primary sensory afferents for the remainder of the body lie in the dorsal root ganglia of the spinal column. The primary sensory afferents and their processes have been classified histologically; the cell bodies fall into two classes: A-type are large (60-120 μm in diameter) while B-type are smaller (14-30 μm) and more numerous. Similarly the processes fall into two categories: C-fibres lack the myelin sheath that A-fibres possess. A-fibres can be further sub-divided into Aβ-fibres, that are large diameter with well developed myelin, and Aδ-fibres, that are thinner with less well developed myelin. It is generally believed that Aβ-fibres arise from A-type cell bodies and that Aδ- and C-fibres arise from B-type cell bodies. These classifications can be further extended and subdivided by studying the selective expression of a range of molecular markers.
Functional analyses indicate that under normal circumstances Aβ-fibres transmit the senses of touch and moderate temperature discrimination, whereas the C-fibres are mainly equivalent to the polymodal high-threshold fibres mentioned above. The role of Aδ-fibres is less clear as they seem to have a variety of responsive modes, with both high and low thresholds.
After the activation of the primary sensory afferents the next step in the transduction of sensory signals is the activation of the projection neurons, which carry the signal to higher parts of the central nervous system such as the thalamic nuclei. The cell bodies of these neurons (other than those related to the cranial nerves) are located in the dorsal horn of the spinal cord. This is also where the synapses between the primary afferents and the projection neurons are located. The dorsal horn is organised into a series of laminae that are stacked, with lamina I being most dorsal followed by lamina II, etc. The different classes of primary afferents make synapses in different laminae. For cutaneous primary afferents, C-fibres make synapses in laminae I and II, Aδ-fibres in laminae I, II, and V, and Aβ-fibres in laminae III, IV, and V. Deeper laminae (V-VII, X) are thought to be involved in the sensory pathways arriving from deeper tissues such as muscles and the viscera.
The predominant neurotransmitter at the synapses between primary afferents and projection neurons is glutamate, although importantly the C-fibres contain several neuropeptides such as substance P and calcitonin-gene related peptide (CGRP). A-fibres may also express neuropeptides such as neuropeptide Y under some circumstances.
The efficiency of transmission of these synapses can be altered via descending pathways and by local interneurons in the spinal cord. These modulatory neurons release a number of mediators that are either inhibitory (e.g. opioid peptides, glycine) or excitatory (e.g. nitric oxide, cholecystokinin), to provide a mechanism for enhancing or reducing awareness of sensations.
A category of sensation that requires such physiological modulation is pain. Pain is a sensation that can warn of injury or illness, and as such is essential in everyday life. There are times, however, when there is a need to be able to ignore it, and physiologically this is a function of, for example, the opioid peptides. Unfortunately, despite these physiological mechanisms, pain can continue to be experienced during illnesses or after injuries long after its utility has passed. In these circumstances pain becomes a symptom of disease that would be better alleviated.
Clinically, pain can be divided into three categories: (1) Acute pain, usually arising from injury or surgery that is expected to disappear when that injury is healed. (2) Chronic pain arising from malignant disease; the majority of people with metastatic cancer have moderate to severe pain and this is resolved either by successful treatment of the disease or by the death of the patient. (3) Chronic pain not caused by malignant disease; this is a heterogeneous complaint, caused by a variety of illnesses, including arthritis and peripheral neuropathies, that are usually not life-threatening but which may last for decades with increasing levels of pain.
The physiology of pain that results from tissue damage is better understood than that which is caused by central nervous system defects. Under normal circumstances the sensations that lead to pain are first transduced by the Aδ- and C-fibres that carry high threshold signals. Thus the synapses in laminae I and II are involved in the transmission of the pain signals, using glutamate and the peptides released by C-fibres to produce activation of the appropriate projection neurons. There is, however, evidence that in some chronic pain states other A-fibres (including Aβ-fibres) can carry pain signals, and thus act as primary nociceptive afferents, for example in the hyperalgesia and allodynia associated with neuropathic pain. These changes have been associated with the expression of peptides such as neuropeptide Y in A fibres. During various chronic pain conditions the synapses of the various sensory afferents with projection neurons may be modified in several ways: there may be changes in morphology leading to an increase in the number of synapses, the levels and ratios of the different peptides may change, and the sensitivity of the projection neuron may change.
Given the enormity of the clinical problem presented by pain, considerable effort has been expended in finding methods for its alleviation. The most commonly used pharmaceuticals for the alleviation of pain fall into two categories: (1) Non-steroidal anti-inflammatory drugs (NSAIDs), including aspirin and ibuprofen; (2) Opioids, including morphine.
NSAIDs have their main analgesic action at the periphery by inhibiting the production of prostaglandins by damaged tissues. Prostaglandins have been shown to be peripheral mediators of pain and inflammation and a reduction in their concentration provides relief to patients. This is especially the case in mild arthritic disease, where inflammation is a major cause of pain. It has been suggested that prostaglandins are involved in the mediation of pain in the spinal cord and the brain; this may explain why NSAIDs have analgesic effects in some pain states that do not involve inflammation or peripheral tissue damage. As prostaglandins, however, are only one of several mediators of pain NSAIDs alone are only effective in reducing some types of mild pain to acceptable levels. They are regarded as having a ceiling of activity above which increasing doses do not give increasing pain relief. Furthermore they have side effects that limit their usefulness in chronic complaints. The use of NSAIDs is associated with irritation of the gastro-intestinal tract and prolonged use may lead to the development of extensive ulceration of the gut. This is particularly true in elderly patients who form the largest cohort of patients with, for example, arthritis.
Opioids act at the level of the spinal cord to inhibit the efficiency of neurotransmission between the primary nociceptive fibres (principally C-fibres) and the projection neurons. They achieve this by causing a prolonged hyperpolarization of both elements of these synapses. The use of opioids is effective in alleviating most types of acute pain and chronic malignant pain. There are, however, a number of chronic malignant pain conditions which are partly or completely refractory to opioid analgesia, particularly those which involve nerve compression, e.g. by tumour formation. Unfortunately opioids also have unwanted systemic side-effects including: (1) depression of the respiratory system at the level of the respiratory centres in the brain stem; (2) the induction of constipation by a variety of effects on the smooth musculature of the gastro-intestinal tract; and (3) psychoactive effects including sedation and the induction of euphoria. These side effects occur at doses similar to those that produce analgesia and therefore limit the doses that can be given to patients.
Delivery of opioids at the spinal level can reduce the side-effect profile, but requires either frequently repeated spinal injections or fitting of a catheter, both of which carry increased risk to the patient. Fitting of a catheter requires that the patient is essentially confined to bed thus further restricting their quality of life.
The use of opioids for the treatment of some other types of chronic pain is generally ineffective or undesirable. Examples include the pain associated with rheumatoid arthritis and neuromas that develop after nerve injury. The undesirable nature of opioid treatment in these patients is related not only to side-effects already mentioned and the probable duration of the disease but also to the fourth major side-effect of the opioids: dependence. Opioids such as morphine and heroin are well-known drugs of abuse that lead to physical dependence, this last side-effect involves the development of tolerance: the dose of a drug required to produce the same analgesic effect increases with time. This may lead to a condition in which the doses required to alleviate the pain are life-threatening due to the first three side-effects.
Although NSAIDs and opioids have utility in the treatment of pain there is general agreement that they are often not appropriate for the adequate treatment of pain, particularly chronic and severe pains.
Other treatments are also used, particularly for the treatment of chronic severe pain including surgical lesions of the pain pathways at several levels from peripheral nerves through dorsal root section and cordotomy to pituitary destruction. These are, however, mostly severe operations that are all associated with significant risk to the patient.
It can be seen, therefore, that there remains a significant need for the development of new classes of pharmaceuticals for the treatment of pain of many types. The desired properties of such new therapies can be briefly expressed as follows: (1) the ability to provide significant relief of pain including severe pain; (2) the lack of systemic side effects that significantly impair the patient's quality of life; (3) long-lasting actions that do not require frequent injections or long-term catheterisation of patients; (4) provision of agents that do not lead to tolerance and associated dependence.